The last few decades have seen significant efforts expended in exploring possible therapeutic uses of D- and L-purine nucleoside analogs. A number of nucleoside analogs are currently being marketed as antiviral drugs, including the HIV reverse transcriptase inhibitors (AZT, ddI, ddC, d4T, and 3TC).
A variety of D- and L-purine nucleoside analogs have also been explored in search of immunomodulators. Guanosine analogs having substituents at the 7- and/or 8-positions, for example, have been shown to stimulate the immune system. See Reitz et al, J. Med. Chem., 37, 3561-78 (1994); Michael et al., J. Med. Chem., 36, 3431-36 (1993). In other research, U.S. Pat. No. 5,821,236 to Krenitsky et al. discloses 6-alkoxy derivatives of arabinofuranosyl purine derivatives that are useful for tumor therapy. Also reported in U.S. Pat. No. 5,539,098 to Krenitsky et al. are inhibitors of varicella zoster virus, including 5′-O-proprionyl and 5′-O-butyryl esters of 2-amino-6-methoxy-9-(β-D-arabinofuranosyl)-9H-purine. 7-Deazaguanosine and analogs have been shown to exhibit antiviral activity in mice against a variety of RNA viruses, even though the compound lacks antiviral properties in cell culture. 3-Deazaguanine nucleosides and nucleotides have also demonstrated significant broad spectrum antiviral activity against certain DNA and RNA viruses. Revankar et al., J. Med. Chem., 27, 1489-96 (1984). Certain 7- and 9-deazaguanine C-nucleosides exhibit the ability to protect against a lethal challenge of Semliki Forest virus. Girgis et al., J. Med. Chem., 33, 2750-55 (1990). Selected 6-sulfenamide and 6-sulfinamide purine nucleosides are disclosed in U.S. Pat. No. 4,328,336 to Robins et al. as having demonstrated significant antitumor activity.
Certain pyrimido[4,5-d]pyrimidine nucleosides are disclosed in U.S. Pat. No. 5,041,542 to Robins et al. as being effective in treatment against L1210 in BDF1 mice. These particular nucleosides were suggested to be as a result of their role as immunomodulators. See Bonnet et al., J. Med. Chem., 36, 635-53 (1993). Also, Wang et al. (WIPO International Publication No. WO 98/16184) report that purine L-nucleoside compounds and analogs thereof were used to treat an infection, infestation, a neoplasm, an autoimmune disease, or to modulate aspects of the immune system. In addition, 3-β-D-ribofuranosylthiazolo[4,5-d]pyrimidines demonstrating significant immunoactivity, including murine spleen cell proliferation and in vivo activity against Semliki Forest virus, are disclosed U.S. Pat. Nos. 5,041,426 and 4,880,784 to Robins et al.
One possible target of immunomodulation involves stimulation or suppression of Th1 and Th2 lymphokines. Type I (Th1) cells produce interleukin 2 (IL-2), tumor necrosis factor (TNFα) and interferon gamma (IFNγ) and they are responsible primarily for cell-mediated immunity such as delayed type hypersensitivity and antiviral immunity. Type 2 (Th2) cells produce interleukins, IL-4, IL-5, IL-6, IL-9, IL-10, and IL-13 and are primarily involved in assisting humoral immune responses such as those seen in response to allergens. See, e.g., Mosmann, Annu. Rev. Immunol, 7, 145-73 (1989). D-guanosine analogs have been shown to elicit various effects on lymphokines IL-1, IL-6, INFα and TNFα (indirectly) in vitro (Goodman, Int. J. Immunopharmacol, 10, 579-88 (1988); U.S. Pat. No. 4,746,651 to Goodman) and in vivo (Smee et al., Antiviral Res., 15, 229 (1991); Smee et al., Antimicrobial Agents and Chemotherapy, 33, 1487-92 (1989)). However, the ability of the D-guanosine analogs such as 7-thio-8-oxoguanosine to modulate Type 1 or Type 2 cytokines directly in T cells was ineffective or had not been described.
Moreover, it is known that the oral administration of many purine nucleoside analogs are subject to difficulties arising from poor absorption, poor solubility, or degradation in the digestive tract as a result of acidic or alkaline conditions or the action of enzymes, and/or combinations of these phenomena. Thus there remains a need for purine nucleoside analogs with improved oral availability, tolerability, and administration that are used to modulate aspects of the immune system.